Protective circuits for electronic equipment



Nov. 23, 1948.

J. M w. STONE 2,454,618

PROTECTIVE CIRCUITS FOR ELECTRONIC EQUIPMENT Filed Feb. 19, 1945 /5 w L9 erzog T 7 6%[4 /9 7 4 Gazer-19757 Patented Nov. 23, 1948 PROTECTIVECIRCUITS FOR ELECTRONIC EQUIPMENT John McWilliams Stone, St. Charles,111., assignor to Operadio Manufacturing 00., St. Charles, 111., acorporation of Illinois Application February 19, 1945, Serial No.578,677

14 Claims. 1

More particularly, this invention relates to means for protecting highfrequency oscillators of the type used to heat plastic and othermaterials for molding, glue setting, etc., it bein the general object ofthe invention to provide a new and improved means for protecting suchoscillators and the electrodes thereof.

Many applications have been made of high or radio frequency currents indielectric heating devices for heating plastic or other materials.Usually to heat the material it is placed between two electrodes whichcarry a high frequency current and form a portion of a high frequencyoscillator output circuit. Since a high voltage exists between theelectrodes, ionization of the air and formation of an arc therebetweenfor frequent occurrences.

If the air gap between the electrodes is broken down so that an areforms, damage as a rule results to the electrodes, the work in process,the oscillator and other parts of the equipment. Frequently, theelectrodes are in the form of relatively expensive dies so that burningor pitting on the surface thereof spoils the dies for precision work.The usual protective devices, such as circuit breakers and fuses, failto give adequate protection in circuits of this nature principallybecause of the time consumed for them to operate.

It is another object of this invention to provide a protective means forhigh frequency power circuits which will disconnect the high frequencypower from the circuit in a fraction of a second after or substantiallysimultaneously with the formation of an arc, and thereby prevent damageto the equipment.

It is another object of the invention to provide a protective means fora high frequency oscillator which utilizes a low frequency or directcurrent carried by the are for actuating a circuit breaking relay.

A more particular object is to provide a disconnecting relay in theprimary circuit of a high frequency oscillator and an operating coil forthe relay connected in series with a low frequency A. C. (or a D. C.)source and the electrodes so that upon the formation of an are betweenthe electrodes by the high frequency current the arc provides a path forcarrying the low frequency current to energize the relay coil.

Other objects will become readily apparent from the following detaileddescription taken in connection with the accompanying drawing wherein,

The figure is a diagrammatic representation of a preferred form of theinvention.

While the invention is herein disclosed as embodied in a protectivemeans for a high frequency oscillator, it is not intended to limit theinvention to the specific form and arrangement shown, it beingcontemplated that various changes may be made by those skilled in theart without departing from the spirit and scope of the appended claims.

As illustrated in the drawing, an oscillator is shown comprising a highfrequency generator l0, coupled to a high frequency oscillating circuitH by means of a coupling link l2. The tank circuit of the high frequencyoscillator includes an inductance l3 and a capacity 14. An electrode !5is connected by means of a lead ll to one side of the tank circuit, andan electrode i6 is connected to the other side by means of leads !8 andI8 and a blocking condenser l9 therebetween. This condenser presentsnegligible impedance to the flow of high frequency current, but blocksthe flow of low frequency A. C. or D. C. current. The electrodes aresuitably arranged in spaced relation so that the work piece or materialW may be positioned therebetween.

The primary of the high frequency generator may receive its energysupply from a 60 cycle A. C. source 26, one side 2| from the primaryleading directly to the energy supply source and the other side 22leading to one contact 23'of a relay, the other contact 24 of which isconnected by a lead 25 to the other side of the energy supply source 20.A relay coil 26 has an armature 21 associated therewith, which armatureis connected to the movable contact 24 at 28 so that when the relay coil26 is energized, the armature 21 is raised and the contacts 23, 24 aredisengaged to break the primary circuit to the high frequency generator.

One end of the coil 26 is connected by a lead 29 to one side of a lowfrequency A. C. or D. C. power source 50 of control current. The otherend of the coil 26 is connected by a lead 30 and a choke coil 3| to theelectrode lead l8 on one side of the blocking condenser H! which isinserted between the leads [8 and IS. A similar choke coil 33 isconnected between the lead I8 (011 the other side of the blockingcondenser i9) and a lead 34 (which connects with the other side ofsource 50) by a lead 35 so that the power source for supplying controlcurrent has high impedance to oscillator currents. By-pass condensers 45and 46 are connected in series between leads 30 and 35 and to ground bya lead 47, and cooperate with the choke coils 3| and 33 to confine flowof the high frequency current to the oscillator circuit and to preventflow thereof in the relay and supply circuits. Thus, a circuit can betraced from the control current source 50 through leads 34 and 35, andchoke coil 33 to the lead l8 on the right hand side of the blockingcondenser and thence through the induction I3 and lead I! to the upperelectrode 15. From the lower electrode l6 the circuit continues throughlead l8 3 on the left hand side of the blocking condenser, choke 3i andleadtli to the coil 26 and thence through lead 29 to source Eli. Duringnormal operation of the high frequency oscillator no control current canflow in this circuit due to the gap between the electrodes i5 and [6'since the power source for supplying control current provides adifference of potential too low to initiate breakdown between the spacedelectrodes l5 and I6, with the result that the relay contacts 23'and 24remain closed by gravity action on armature 2i, and the equipmentremains in operation. If, however, an are forms between the electrodesl5 and iii, the arc completes a low impedance path for the controlcurrent between the electrodes, and flow of current from the powersource 5f: energizes the relay coil 2i; and causes it to raise thearmature 21, opening the contacts 23, 2 1, and shutting down ordisabling the equipment by cutting off the energy supply 20. The controlcurrent flows substantially instantaneously upon formation of an arcwith the result that the low frequency power supply to the oscillator isshut off before any damage can occur.

Preferably, a manual switch 36 is provided in a holding circuit for therelay coil so that after the relay has been operated to shut off thehigh frequency current to the electrodes, it is necessary to open theswitch 36 before the equipment can again be placed in operation. Theswitch has a stationary contact 31 connected by a lead 38 to lead 30 atone end of the relay coil. The movable contact of the switch isconnected by a lead 39 to a movable contact 40 carried on the relayarmature 21. Normally the contact 40 engages a stop member 4|, but whenthe relay is energized, it engages astationary contact 42 which isconnected by a lead 43 to the junction of leads 34 and 35. Thus, whenthe relay coil is energized and the armature 21 is raised a holdingcircuit for the relay coil is completed through the contacts 41], 42,and in order to place the equipment in operation again it is necessaryto disengage the manual switch 36.

It is contemplated that the relay coil 26 may be selectedfor operationfrom a low frequency source of A. C. similar to the source or that itmay alternatively be selected for operation on direct current. Whicheverthe variety of control current provided by the source 50 it is intendedthat it be of such type as will not flow across the gap between theelectrodes l5 and I6 except when carried by an are or otherobjectionable discharge therebetween of the high frequency current.

I claim:

1. In combination, a high frequency oscillator including a primarycircuit, a tank circuit and output electrodes deriving energy from saidtank circuit, means for controlling the operation of said oscillatorcomprising a relay having a pair of contacts in said primary circuit, anoperating coil for said relay which when energized disengages saidcontacts and shuts down the oscillator,

a blocking condenser inserted in series with one electrode and the tankcircuit presenting negligible impedance to the flow of high frequencycurrent but blocking the flow of low frequency alternating current, acircuit for energizing said relay coil comprising a source of lowfrequency alternating control current, a connection between said sourceand said oscillator at one side of said blocking condenser, a connectionfrom the other side of said source to one endof said coil and a;connection from the other end of said coil to said the ends of saidradio frequency chokes remote from the oscillator, the circuit throughthe relay operating coil being completed upon an are discharge betweenthe electrodes of the high frequency circuit which are dischargeprovides a conducting path between the electrodes for the low frequencycurrent and causes energization of the relay and shuts down theoscillator.

2. A combination as defined in claim 1 including means operated by saidcoil to form a holding circuit therefor when energized, and meansoperable to break said holding circuit to reco-ndition the highfrequency oscillator for operation.

3. In combination, a high frequency oscillator including a primarycircuit, a tank circuit and output electrodes deriving energ from saidtank circuit, means for controlling the operation of said oscillatorcomprising a relay for said primary circuit, said relay having anoperating coil which when energized shuts down the oscillator, ablocking condenser inserted in series with the electrodes and tankpresenting negligible impedance to the flow of high frequency currentbut blocking the flow of control current, a circuit for energizing saidrelay coil comprising a source of control current, a connection betweensaid source and said oscillator at one side of said blocking condenser,a connection from the other side of said source to one end of said coiland a connection from the other end of said coil to said oscillator onthe other side of said blocking condenser,

means for preventing high frequency current from passing through saidoperating coil, the circuit through the relay operating coil beingcompleted upon an arc discharge between the electrodes of the highfrequency circuit which arc discharge provides a conducting path betweenthe electrodes for the control current and causes energization of therelay and shuts down the oscillator.

4. A combination as defined in claim 3 including means operated by saidcoil to form a holding circuit therefor when energized, and meansoperable to break said holdin circuit to recondition the high frequencyoscillator for operation.

5. In combination, a high frequency oscillator including a primarycircuit, a tank circuit and output electrodes deriving energy from saidtank circuit, means for controlling the operation of said oscillatorcomprising a relay for said primary circuit, said relay having anoperating coil which when energized shuts down the oscillator, ablocking condenser inserted in series with the electrodes and tankpresenting negligible impedance to the flow of high frequency currentbut blocking the flow of direct current, a circuit for energizing saidrelay coil comprising a direct current source, a connection between saidsource and said oscillator at one side of said blocking condenser, aconnection from the other side of said source to one end of said coil, aconnection from the other end of said coil to said oscillator on theother side of said blocking condenser, radio frequency chokes in saidconnections to the opposite sides of said blocking condenser, by-passcondensers connected between the ends of said radio frequency chokesremote from the oscillator, the circuit through the relay operating coilbeing completed upon an arc discharge between the electrodes of the highfrequency circuit which are discharge provides a conducting path betweenthe electrodes for the direct current and causes energization of therelay and shuts dow the oscillator.

6. In combination with a high frequency oscillator having an energysupply circuit, an output circuit, and output electrodes deriving energyfrom said output circuit, means for controlling the operation of theoscillator comprising an electrically operated switch device in theenergy supply circuit of the oscillator including an operating coilwhich when energized shuts down the oscillator, a source of controlcurrent, and means connecting said coil in series with said electrodesand across said control current source so that an arc discharge betweenthe electrodes forms a conducting path for the con trol currentcompleting the circuit for energizing the operating coil of the switchdevice.

7. In combination with a high frequency oscillator having an energysupply circuit, an output circuit, and output electrodes deriving energyfrom said output circuit, means for controlling the operation of theoscillator comprising an electrically operated switch device in theenergy supply circuit of the oscillator including an operating coilwhich when energized shuts down the oscillator, a source of lowfrequency control current, means connecting said coil in series withsaid electrodes and across said control current source so that an arcdischarges between the electrodes forms a conducting path for thecontrol current completing the circuit for energizing the operating coilof the switch device, and means for preventing high frequency currentfrom passing through said operating coil.

8. In combination, a high frequency oscillator including a primarycircuit, a tank circuit and output electrodes deriving energy from saidtank circuit, means for controlling the operation of said oscillatorcomprising a relay for said primary circuit, said relay having anoperating coil which when energized shuts down the oscillator, ablocking condenser inserted in series with the electrodes and tankpresenting negligible resistance to the flow of high frequency currentbut blocking the flow of a control current, a circuit for energizingsaid relay coil comprising a control current source, a connectionbetween said source and said oscillator on one side of said blockingcondenser, a connection from the other side of said source to one end ofsaid coil and a connection from the other end of said coil to saidoscillator on the other side of said blocking condenser.

9. In combination with a high frequency oscillator having an energysupply circuit and having output electrodes deriving energy from theoscillator, means for controlling the operation of the oscillatorcomprising an electrically operated switch device in the energy supplycircuit of the oscillator including an operating coil which whenenergized shuts down the oscillator, a source of control current, andmeans connecting said coil in series with said electrodes and acrosssaid control current source.

10. In combination, a high frequency oscillating circuit including ablocking condenser and two spaced electrodes in series, said spacedelectrodes forming part of a load, a power source for supplying controlcurrent, said source having high impedance to oscillator currents andbeing connected to said circuit at points having low impedanceconnections to said spaced electrodes.

said power source providing a difference of potential too low toinitiate breakdown between said spaced electrodes and said spacedelectrodes normally providing an open circuit for said power circuitwhile an are between said electrodes provides a low impedance betweensaid spaced electrodes to close said power circuit, means responsive toflow of current from said power source incident to arcing for disablingsaid oscillating circuit, and means independent of arcing conditions forrestoring said oscillating circuit to operation.

11. Apparatus of the character claimed in claim 10, wherein a lowfrequency energy supply source is connected to the oscillating circuitfor supplying the energy thereto, and wherein said disabling meanscomprises means for cutting off said oscillating circuit from said lowfrequency energy supply.

12. Apparatus of the character claimed in claim 10, wherein a source oflow frequency energy is provided for said oscillating circuit forenergizing said circuit, and wherein said disabling means includes meansenergized by said control power source for maintaining said disablingmeans in a locked condition after disabling said oscillating circuit.

13. Apparatus of the character claimed in claim 10, wherein a lowfrequency energy supply source is provided for the oscillating circuitfor energizing said circuit, and wherein said control current powersource is energized by low frequency, and wherein the disabling meansincludes means energized by one of said low frequency sources formaintaining and disabling means looked after disabling said oscillatingcircuit.

14. In combination, a high frequency oscillating circuit including ablocking condenser and two spaced electrodes in series, said spacedelectrodes forming part of a load, a power source for supplying controlcurrent, said source having high impedance to oscillator currents andbeing connected to said circuit at points having low impedanceconnections to said spaced electrodes, said power source providing adifference of potential too low to initiate breakdown between saidspaced electrodes and said spaced electrodes normally providing an opencircuit for said power source While an are between said electrodesprovides a low impedance between said spaced electrodes to close saidpower source, means responsive to flow of current from said power sourceincident to arcing for disabling said oscillating circuit, and manuallyoperable switch means independent of arcing conditions for restoringsaid oscillating circuit to operation.

JOHN McWILLIAMS STONE.

REFERENCES CITED The following references are of record in the file ofthis patent:

UNITED STATES PATENTS Number Name Date 1,694,837 Walker, Jr Dec, 11,1928 2,049.948 Cuchman Aug. 4, 1936 2 276.643 Bates Mar. 1'7, 19422,289,813 Henke July 14, 1942 2,289.836 Grothe July 14, 1942 2,293.533Deneen Aug. 18, 1942 2,300,127 Maddock Oct. 27, 1942 Certificate ofCorrection Patent No. 2,454,618. November 23, 1948.

JOHN MCWILLIAMS STONE It is hereby certified that errors appear in theprinted specification of the above numbered patent requiring correctionas follows:

Column 1, line 16, for the Word for read are; column 2, line 58, forinduction read inductance; column 5, line 30, claim 7, for dischargesread discharge;

and that the said Letters Patent should be read with these correctionstherein that the same may conform to the record of the case in thePatent Ofiice.

Signed and sealed this 19th day of April, A.,D. 1949.

THOMAS F. MURPHY,

Assistant Commissioner of Patents.

Certificate of Correction Patent No. 2,454,618. November 23, 1948. JOHNMcWILLIAMS STONE It is hereby certified that errors appear in theprinted specification of the above numbered patent requiring correctionas follows:

Column 1, line 16, for the word for read are; column 2, line 58, forinduction read inductance; column 5, line 30, claim 7, for dischargesread discharge;

and that the said Letters Patent should be read with these correctionstherein that the same may conform to the record of the case 1n thePatent Ofiice.

Signed and sealed this 19th day of April, A. D. 1949.

THOMAS F. MURPHY,

Assistant Uommissz'oner of Patents.

